The present invention relates to a method to transparently transport an incoming clock signal over a network segment and to a transmitting unit and a receiving unit equipped to perform this method.
As is well known in the art, such a method is to be used for instance in telecommunication networks wherein a network timing reference signal is to be transported over the network but wherein data are transported over a network segment synchronised to a timing reference signal internal for this network segment. The network timing reference signal has to be transmitted over this network segment although it may not be used therein. Within a segment of an ATM (Asynchronous Transfer Mode) network, data may for instance be transmitted over a telephone line in accordance with the ADSL (Asymmetric Digital Subscriber Line) specifications. The transmission of data packed in ADSL frames over the telephone line between a transmitting and a receiving modem is synchronised to the modem clocks. Nevertheless, network layer specifications require that the ATM network timing reference signal be transparently transported over this ADSL network segment. From the point of view of the network segment, the ATM network timing reference signal thus is an incoming clock signal which unaffectedly has to appear at the exit of the ADSL network segment, i.e. at the output of the receiving modem. This could be done by sending the network timing reference signal over a separate transmission means or over the telephone line thereby using part of the transmission capacity of this line. Moreover, this way of transmitting the network timing reference signal implies a considerable complexity increase for the transmitter and receiver.
An object of the present invention is therefore to realise the transmission of the timing reference signal in an efficient way, i.e. without huge complexity increase of the transmitter and receiver in the network segment over which the clock signal is to be transported.
The object is also realized by a transmitting unit to a first input of which data are applied and to a second input of which an incoming clock signal is applied, the transmitting unit including: embedding means, coupled between the first input and an output of the transmitting unit, and adapted to embed the data in data frames and to apply a data frame to the output of the transmitting unit upon triggering by a reference signal derived from a transmit clock signal applied to a clock input of the transmitting unit by a transmit clock, wherein the transmitting unit further includes: phase measurement means to a first and a second input of which the incoming clock signal and the reference signal are applied respectively, and which is adapted to measure a phase difference value between the incoming clock signal and the reference signal, and to apply the phase difference value to an output of the phase measurement means; and further that the embedding means is provided with an additional input terminal connected to the output of the phase measurement means, the embedding means further being adapted to embed the phase difference value in a data frame.
The object is still further realized by a receiving unit to an input of which data frames are applied, the receiving unit including: de-embedding means with an input coupled to the input of the receiving unit, the de-embedding means being adapted to retrieve data to a first output of the receiving unit, and to retrieve a phase difference value out of a reserved field of the data frames and to apply the phase difference value to a phase output of the de-embedding means, wherein the receiving unit further includes: generating means, to whose first input connected to the phase output the phase difference value is applied, and to whose second input a second reference signal obtained from a receive clock signal applied to a clock input of the receiving unit by a receiver clock is applied, the generating means being adapted to generate an outgoing clock signal equal to the well-known frequency and with a phase difference compared to the second reference signal equal to the phase difference value.
According to the invention, this object is realised by a method to transparently transport an incoming clock signal with a well-known frequency over a network segment consisting of a transmitting unit to an input of which the incoming clock signal is applied and to a clock input of which a transmit clock signal is applied by a transmitter clock, a transmission medium, and a receiving unit to a clock input of which a receive clock signal is applied by a receiver clock synchronised with the transmitter clock, wherein the method includes the steps of measuring a phase difference value between the incoming clock signal and a reference signal obtained from the transmit clock signal; transmitting the phase difference value from the transmitting unit to the receiving unit; and generating in the receiving unit an outgoing clock signal with a frequency equal to the well-known frequency, the outgoing clock signal having a phase difference with a second reference signal, similarly obtained from the receive clock signal as the reference signal is obtained from the transmit clock signal, equal to the phase difference value.
Indeed, since transmission over the network segment is synchronised to transmit the clock signal, and since both clock signals, the transmit clock signal and receive clock signal, are synchronised, the receiving unit only has to become aware of the phase difference between the incoming clock signal and a reference signal synchronous to the transmit clock signal to be able to generate a copy of the incoming clock signal, provided that it also has a reference signal similarly synchronous to the received clock signal. The reference signal may be obtained by frequency dividing the transmit clock signal. Obviously, a similar reference signal obtained by frequency dividing the receive clock signal then has to be used at the receiver""s side in combination with the measured phase difference value to generate the outgoing clock signal there. Determining the phase difference and using it in the receiver and generating a reference signal obviously requires less additional complexity in the transmitter and receiver than would be needed using known methods.
In a particular implementation of the present method wherein the additional required complexity is even more reduced, the reference signal equals the data frame clock signals.
In this way, the phase difference value is determined by measuring the time interval between the incoming clock signal and the data frame boundary each time a data frame is transmitted. The phase difference value is measured and transmitted once per data frame. If the data frame is sufficiently large (e.g. an ADSL superframe with a length of 68xc3x97250 xcexcs), the additional overhead due to transmission of the phase difference value from transmitting to receiving unit is negligible. As will be seen later, the phase difference can easily be measured by means of a counter in this particular implementation.
An advantageous feature of this particular implementation is where the phase difference value is measured with a precision equal to one period of the transmit clock signal.
Indeed, as will be described in detail later on in the description, the just mentioned implementation with a counter can be realised so that the phase difference value is measured as an integer amount of transmit clock pulses. Since the transmit clock and the receive clock are synchronous, the phase difference to be realised in the receiving unit will also be an integer amount of receive clock pulses.
A further specific feature of the present method is that the phase difference value may be embedded in fields of the date frames.
In this way, no additional overhead is added to the data frames to transport the phase difference information. This technique is particularly recommended if in the network segment, data are transmitted packed in data frames wherein some fields are reserved for special use.
If the data are transmitted in the network segment in accordance with the Asymmetric Digital Subscriber Line (ADSL) specifications, the phase difference values may occupy fields reserved for so called fast bytes of Discrete Multi Tone symbols.
Indeed, an ADSL superframe contains several fast byte fields only a part of which are used for transporting operation channel related information. Consequently, the remaining fast byte fields may be used to transport the phase difference values.
Another additional feature of the present method is where the phase difference value is transmitted only when it differs from the previously measured and transmitted phase difference value.
Hence overhead occupancy by phase difference values is further reduced by transmitting phase difference values only if they differ from a previous transmitted value. Since the receiver is aware of this previous transmitted value, it can continue generating the outgoing clock signal without precision decrease when it receives no new phase difference values for a certain time period.
In an alternative embodiment, not the phase difference itself but the deviation from the previous phase difference is transmitted. Again, the overhead can be reduced further by transmitting phase difference deviation values only if they differ from a previous transmittal value. This technique is especially advantageous in case of a fixed clock offset of the incoming clock signal relative to the reference signal synchronous to the transmit clock. In this case the phase difference deviations are (almost) constant and thus need not be transmitted.